Optical resonators used in high-energy and high-power applications have resulted in optical structures characterized by a high Fresnel number. This leads to poor mode control and low far-field brightness. Additionally, in a high Fresnel number resonator, some local transient inhomogeneity of the resonator-gain medium system can cause optical energy to become concentrated in hot spots which lead to a degradation of the gain medium and of the optical surfaces. These transient concentrations of energy cannot be readily controlled since the geometry of the resonator is such that smoothing of the energy in the beam due to diffraction is very slow. To overcome these problems, unstable resonators have been used. They provide transverse mode selection by increasing the transverse diffusion of light within the resonator and increases efficiency by using the energy lost to diffraction as the output beam.
A problem of typical unstable resonators in that they give rise to an annular output beam. This produces a low far-field brightness which limits the usefulness of these configurations.
Another problem found in unstable resonators is that the threshold gain of the active medium within such resonators has a complex periodic oscillation as a function of the magnification and other parameters.
Unstable resonators that employ a structure leading to a non-annular output beam have used structures having multiple optic axes which can give rise to a non-uniphase output beam due to the different contributions of each optic axis.
A further problem found with these unstable resonators is the poor volumetric efficiency obtainable as a result of the low volume utilization of the gain medium. Much of the gain medium is not traversed by any portion of the output beam because of the geometry of typical unstable resonator and the gain medium within it.